ES2231668T3 - PROCEDURE TO PREPARE SILICON CARBIDE. - Google Patents

Abstract

Process for preparing silicon carbide and optionally aluminum and silumin (aluminum silicon alloy) in the same cell, wherein; silicate and/or quartz containing rocks are subjected to electrolysis in a salt melt consisting of a fluoride-containing electrolysis bath, whereby silicon and aluminum are formed in the same bath, and formed aluminum, which may be low alloyed, flow towards the bottom and is optionally drawn off, carbon powder from the cathode material and/or from external sources is added directly to the molten bath or frozen bath in addition to the cathode deposit, the frozen bath and the cathode deposit being crushed before or after the addition of carbon particles; the obtained mixture is melted at a temperature above 1420°C, and SiC is crystallized by cooling.

Description

Procedure for preparing carbide silicon.

The present invention relates to a procedure for preparing silicon carbide in a salt melt. Silica and silicate rocks and / or silicate rocks that contain aluminum are used as raw material material, with / without soda (Na2CO3) and / or limestone (CaCO3) dissolved in fluorides, in particular cryolite.

The prepared products are of high purity.

WO 95/33870 (EP 763151), hereinafter referred to as "WO 95", describes a procedure for continuous preparation and batch preparation in one or several stages in one or several furnaces, of silicon (Si), optionally silumin (AlSi alloys) and / or aluminum metal (Al) in a foundry bath using feldspar or rocks that They contain feldspar dissolved in fluoride. In that procedure If high purity is prepared by electrolysis (step 1) in a first furnace with a replaceable carbon anode arranged below of the cathode, and a carbon cathode arranged at the top of the oven. For the preparation of silumin the electrolyte reduced by silicon Stage I residual is transferred to another oven, and Al is added (stage II). Then, Al is prepared in a third oven (stage III) by electrolysis after the Si has been removed in the stage I and possibly stage II. Also describes oven combinations with a partition wall in preparation of the same substances. In addition, the process team is described for the procedure

The present invention represents a development more and the improvement of the aforementioned procedure.

The other most important development is that it prepare high purity SiC in molten Si, as explained in continuation.

A big improvement is that it is possible to prepare Yes pure (which becomes SiC), Alloy alloys slightly alloyed with low purity iron (AlSi alloys) and Al alloys very alloys with low purity phosphorus (SiAl alloys) in it oven (stage 1) varying such parameters as the selection of the Raw material, current density (voltage) and time. The Proportions of Si and Al products are adjusted by the Raw material selection and cathodic current density (voltage) in the electrolysis bath and the mechanical manipulation of The cathodes In addition, the composition of Al- (AlSi alloy) as referenced in this document, it is an Al alloy with an amount of Si which is lower than that of a eutectic mixture (12% of Si, 88% of Al). In proportion, a very alloyed alloy (alloy of SiAl) as indicated in this document is an alloy that has a Si content higher than that of a eutectic mixture.

In accordance with the present invention, provides a process for preparing silicon carbide of according to the characteristics of claim 1.

Soda can be added to the electrolysis bath of so that said bathroom will be basic if quartz is used, to avoid loss of Si in the volatile form of SiF_ {4}. With high soda concentrations the melting point of the mixture is reduced, and the use of added fluorides decreases. Limestone is added if it were necessary to reduce phosphorus absorption in the deposited Si On the cathode

In connection with crystallization fluorides preferably they should be acidic.

A new property according to the invention is that the carbon that it has or that has been taken from the cathode (by example as chips) or from external sources, mixed with silicon (in the electrolyte). The carbon dust can then both be added to the molten bath and / or how to be added to the solid frozen bath and / or to the cathode deposit. If the molten bath or the frozen bath are used the cathode deposit should be scraped in advance in bathroom. The carbon is mixed with the desired fractions and is crush to the desired grain size. The obtained mixture consisting in Si, electrolyte and C either melts directly or preferably it is subjected to acid treatment as described below. If acid treatment is used, powder fusion treated by acid will correspond to stage III. Carbon in any case must be added in stoichiometric excess to get a complete conversion to SiC in the merger procedure of Si, stages II and III.

An advantageous alternative is that it is added H 2 SO 4 concentrated to the deposit of the untreated cathode, sprayed (stage II) containing 20% Si, and / or the bath sprayed (electrolyte) containing 20% Si after the cathode deposit has been scraped in the bath, and carbon. The powder fractions initially cause a Si concentration of approximately 50% because sulfuric acid has a good effect in dissolving the cryolite. This mixture of 50% Si and others residual products, that is, acid sulfates, represent a sticky substance that must be treated later. Diluting the mix with water and adding HCl in dilute amounts during some time a very good release of grains of Si is achieved that float on the surface. The addition of HCl also has the effect of the Si refining, so that the powder mixture is not sticky. In this way it is possible to obtain an increase in the concentration of If together with C in a mixture of Si / C / electrolyte grains with a consistency of sand in water. This consistency of sand in water it has the effect that the mixture is easy to filter and that it wash with water and dry at room temperature. Like a consequence of the increase in the concentration of Si and C in the mixture powder, the use of a hydraulic sorter as a separator (WO 97) becomes superfluous. What happens is that the mixture acid reacts gradually with the electrolyte and dissolves it. The Si grains that are partly embedded in the electrolyte, are released gradually and come into contact with the acid / water mixture. Water acid attacks the contamination in Si and C, which mainly They consist of metals. Hydrogen gas forms on the surface and in the pores of the grains of C and Si, causing an elevation of until very dilute acid. In addition to the fact that Si (d = 2.3 g / cm3) and C (d = 2.1 g / cm3) float on the surface of the water, the grains of C and Si will remain until they are removed away from the surface. Si grain refining has also been improved in addition to concentration, because acids enter better in contact for a longer time with the grains of C and If released. (The grains of C and Si are so pure that they get below the detection limit for all the analyzed elements with the microwave equipment. This means that there is no method  of analysis that can determine if more pure than approximately 99.99% since it is impossible to concentrate Yes at approximately 100% from a mixture of Si / C / electrolyte grains).

Example 1

(From document WO 95)

I. A feldspar of the type CaAl_ {2} Si_ {2} O_ {8} containing 50% of SiO2, 31% of Al 2 O 3 and 0.8% Fe 2 O 3, was dissolved in cryolite and electrolyzed with a cathode current density of 0.05 A / cm2 (U = 2.5-3.0V) for 18.5 hours. At deposit around the cathode formed if highly purified separated from the small grains of FeSi. In the dissolved electrolyte Al 2 O 3 was formed. Al was not formed when the density of Current was so low.

How Al was not formed in the bathroom (the electrolyte containing Al 3+) this was the reason why the bathroom was removed from this oven (stage I) and taken to another oven (stage II) in which the residues of Si and Si (IV) were eliminated by addition of Al before electrolysis and preparation of Al in a third oven (stage III). (See WO 95).

II. The cathode deposit it contained approximately 20% of Si was removed from the cathode. Also the Carbon (graphite, C) of the cathode followed. The mixture obtained from powder was acid washed as described above to obtain a mixture in which the concentration of Si and C had been increased

III. The mixture of II together with the residues of Acid fluorides were melted above 1420º. He does crystallized contained large areas with SiC "highly purified "as demonstrated by the microwave analysis. The sample purity was 99.997% SiC in a matrix that It consisted of 99.997-99.99999% Si.

Conclusion : The reason why not only SiC was obtained was a stoichiometric carbon deficit in the Si melt, stages II-III. The reason why Yes only and not Al was formed in stage I in this case was the low current density (voltage).

Example 2

(Alternative procedure for stage I with formation of To the)

A diorite (rock) that contains feldspar and quartz, analyzed containing 72% SiO2, 16% of Al 2 O 3 and 1.4% Fe 2 O 3, was dissolved in cryolite and electrolyzed at a cathodic current density of 0.5-1.6 A / cm2 (U = 2.5-8.0 V) for 16.5 hours. In the deposit around the cathode they formed Many small grains of highly purified Si and FeSi separated. Al (low alloy AlSi alloy) was formed under the electrolyte and It had a low iron content.

Conclusion : The reason why both Si and Al were formed in stage I was the high current density (voltage). The reason why Al (AlSi alloy) has a low iron content was that FeSi grains remained in the cathode deposit.

No carbon was added in the experiment. For the so much could not form SiC.

It should be expected that if carbon is added to the Si regularly purer, the purity of the resulting SiC will also be increased

Example 3

(Alternative procedure for stage I with formation of To the)

Quartz containing about 99.9% was dissolved SiO2 in cryolite (Na3 AlF6), was mixed with soda 5% (Na2CO3) and electrolyzed with a density of cathode current of 0.5 A / cm2 (U = 6-7 V) for 44 hours Yes was formed in the deposit around the cathode highly purified Most (12 kg) of the cathode deposit  It was pushed into the bathroom (the electrolyte). The deposit of remaining cathode (8 kg) was taken out with the cathodes together with the anode residues The cathode deposit was easily removed of the cathodes and was mixed with the electrolyte in the bathroom. Both of them They contained 20% of Si. Small amounts of Al (AlSi alloy little alloyed) were formed, which was low in iron and phosphorus. The AlSi alloys poor in iron and phosphorus are defined as <130 ppm Fe and <8 ppm P. The Al analysis showed 8% of Si, 110 ppm of Fe and 0.08 ppm of P. The crystallized silicon it contained a total contamination of 3 ppm corresponding to 99.9997% of Si.

Conclusion : The reason why both Si and Al were formed in stage I was the high current density (voltage). Al came from electrolyzed cryolite. The reason why Al (the AlSi alloy) was now very much altered with Si, was that the Si of the cathode deposit began to dissolve in Al. The reason why Al's alloy was poor in iron and phosphorus was that The raw materials were initially low in iron and phosphorus.

The silicon together with the small waste FeSi grains prepared by the acid refinement described previously without the addition of carbon, it contains a total of 75 ppm of Fe and approximately 15 ppm of P. Si powder mixture concentrated contained 80% Si or more than 80% Si. After the Si crystallization of the Si melt, the Si contained only Contamination at 3.0 ppm. A mixture of slag with fluorides Si acids will promote the formation of an even purer SiC. In a possible crystallization of SiC from a refined Si melt of fluoride that also contains carbon, you have to wait for a content even lower contamination in SiC than the one shown in the Example 1.

If you want to prepare Al together with Si, the density of cathodic current must be relatively high, at least higher at 0.05 A / cm2, preferably greater than 0.1, in particular greater than 0.2 A / cm2. An upper limit is approximately 2, preferably about 1.6 A / cm2. Besides the formation of aluminum with a high current density, the electrolysis rate also increases with increasing cathode current density.

With electrolysis it was found that purity of Si was in the 99.92-99.99% range. Previously, (document WO95), to concentrate Si a approximately 20% in addition to the cathode deposit, the deposit of the cathode was crushed as much as possible so that both free and partially non-Si grains float above and could be picked up on the surface in a heavy liquid consisting of different mixtures of C 2 H 2 Br 4 / acetone with a density of up to 2.96 g / cm3. Si in solid form has a density of 2.3 g / cm3 and will float above, while the solids of Cryolite have a density of 3 g / cm3 and will remain in the background. After filtration and drying of the powder for heavy liquid removal, concentration fractions different were mixed with water / H 2 SO 4 / HCl to refine he does.

In WO97 / 27143, hereinafter designated as "WO 97", water, HCl and H 2 SO 4 to the crushed cathode tank, which contained 20% Si, to refine the Si with dilute NaOH that was formed by adding Water. Then he tried to concentrate the powder that contained Si refined with HCl, with concentrated H2SO4.

Neither in WO 95 nor in WO 97 the Si was concentrated more than about 40%. The reason for this is that they were hydrolyzed fluorooxosilicate complexes in the tank of cathode in water and NaOH to form a hardly soluble silica hydrated As a consequence of this, an addition of H 2 SO 4 after the treatment with the water did not achieve the effect of concentration that is taken when added directly to the powder dry untreated The concentrated HCl has no effect of essential concentration because it contains a lot of water in contrast to the concentrated H 2 SO 4. In WO 97 a sorter was used hydraulic to concentrate more Si. This results in only one insignificant concentration

When it is mainly desired to prepare SiC, it use a rock that contains quartz properly as a material of departure. If Al is also of interest, a rock containing a feldspar rich in Al, for example anortite (CaAl_ {2} Si_ {2} O_ {8}).

Si can be fused together with Al prepared in electrolysis (stage 1), to form AlSi alloys little poor alloys in Fe and poor in P, and / or SiAl alloys very alloys that are desired alloys in many connections.

Both very alloyed SiAl alloys and Alloy alloys with low alloys can be dissolved in HCl or H_ {SO} {4}. Al goes into the solution and Si powder forms "pure" (approximately 100% Si), without electrolyte. Starting from the dissolved Al pure AlCl 3 products are formed and Al 2 (SO 4) 3.

As regards the equipment, it is appropriate that the walls consisting of graphite in the electrolysis furnace can be advantageously substituted by SiC or SiC bound to nitride of silicon.

The walls of the electrolysis oven do not have which consist of Si (WO 95, figure 2 number 4). Further, If you do not have to cover the trunk of the anode, because a power jump between the cathode and the anode, even when they grow  together.

Claims (7)

1. A procedure for preparing carbide silicon in which:

I.

silicate and / or rocks containing quartz undergo electrolysis in a salt melt consisting of a electrolysis bath containing fluoride, by which it is formed silicon in the same bathroom and in a cathode deposit,

II.

be add carbon powder from the cathode material and / or sources external directly to the molten bath or the frozen bath in addition to the cathode deposit, the frozen bath and the cathode deposit before or after the addition of particles of carbon;

III.

be add concentrated sulfuric acid and then hydrochloric acid and water to the product of stage II;

IV.

the mixture obtained from the released Si grains and particles of carbon floating on the surface along with the slag melts at a temperature above 1420 ° C and SiC crystallizes cooling

2. The procedure according to the claim 1, wherein both the anode and the cathode are prepared from graphite, and the anode is placed under the cathode. 3. The procedure according to the claim 2, wherein an amount of graphite is removed from the cathode and / or is added externally such that the amount of carbon is greater than the stoichiometric amount of carbon in Sic. 4. The procedure according to any of claims 1-3, wherein the bath of Electrolysis containing fluoride comprises cryolite. 5. The procedure according to any of claims 1-4, wherein soda is used (Na 2 CO 3) and limestone (CaCO 3) in the bath of electrolysis. 6. The procedure according to any of claims 1-5, wherein rocks are used containing quartz as a starting material. 7. The procedure according to any of claims 1-6, wherein a basic, neutral or preferably acidic electrolyte containing fluoride in stage III in the mixture of molten silicon powder and of carbon, which gradually crystallizes in SiC.